This invention generally relates to electronic amplifier circuits and more particularly to an amplifier with .a fast recovery time.
Amplifiers are commonly used to provide gain to an electric signal. For example, if a voltage amplifier has a voltage gain of 10, then an input signal of 50 millivolts (xe2x80x9cmVxe2x80x9d) applied to the voltage amplifier results in an output signal of 500 mV. An amplifier typically has a range in which the amplifier operates in a linear manner. For example, a voltage amplifier connected to a 5-volt power supply may be linear for outputs up to 4.5 volts. However, driving output voltages greater than 4.5 volts may force the amplifier into non-linearity, resulting in distortion from the amplifier. Thus, a problem may develop when the input signal multiplied by the gain of the amplifier exceeds the output capability of the amplifier. For example, an input signal greater than 450 mV in the above-described voltage amplifier (with a voltage gain of 10) may result in distortion of the output signal of the amplifier.
When the linear range of an amplifier is exceeded, some of the internal nodes of the amplifier may be driven beyond their normal operating range, causing the amplifier to operate in a non-linear mode. When an amplifier operates above its designed linear range, non-linearities, distortion, and instability in the output signal may result. Moreover, even after the amplifier returns to its linear operating range, it may take some time for the device to recover and resume operating in its normal state. Such a recovery period, which may take several or even hundreds of nanoseconds, may be unacceptable when a device is used in high-frequency applications, particularly if the amplifier may be required to respond to input signals at a very high rate.
For some applications, these recovery limitations are not critical, because the input voltages can be limited to the linear region. However, in certain applications, the range of input voltages can be very wide. For example, in certain ultrasound applications, sound waves are transmitted into a human body and the reflected echo is detected and converted to an electrical output, e.g., an output that can be displayed on a video monitor. A large object within the body may result in a high-amplitude signal being applied to the ultrasound sensor. The ultrasound system may need to recover from the large input signal before it can effectively resolve other signals. However, because an ultrasound system typically operates in real-time, such a delay is undesirable as it may result in the non-detection of, e.g., a small tumor, because of the nearby presence of a large object.
Presently known systems have addressed this problem by either clamping the input signal or the output signal from an amplifier. However, such a configuration may not be desirable for various reasons. For example, high gain amplifiers may overload with even relatively small input signals, making it difficult to integrate a clamping network for low-level input signals. Clamping at the output port may be easier to implement, but may not prevent the various internal sections of an amplifier from overloading in response to high amplitude input signals. Clamping at the output port may also lead to instability or even oscillation because the loop gain of the amplifier may be dynamically changed when the clamp function is activated.
An amplifier control circuit is thus desired which overcomes the shortcomings of the prior art.
An amplifier with a network used to clamp the output amplifier to prevent an overload condition is disclosed. The amplifier may also include an electrically controllable gain function exhibiting enhanced protection against overload. The amplifier circuit contains a buffer amplifier that converts an input voltage signal to a current signal and an output amplifier that converts a current signal to an output voltage signal. An internal resistance that can be electrically configured to various desired levels may control the gain of the amplifier.
Also disclosed is a method of amplifying an input signal. The voltage of the input signal is sensed and the input signal is converted to a current signal. A transimpedance amplifier converts the current signal into an output voltage signal. The output voltage signal is clamped if the voltage exceeds a predetermined value. The conversion of the voltage signal to a current signal may encompass the use of a resistor. This conversion may be configured to depend on the resistance value. Furthermore, the resistance may be varied to control the gain of the amplification.